SELLING a made-in-China garment to a Hong Kong middleman at an artificially low price, then invoicing it on at the correct value is a neat way for a mainland company to build up its foreign exchange reserves. Unfortunately, it plays havoc with the calculations in China's balance of payments statistics. Unless someone is brave enough to put it forward, this is one of the economic items least likely to be on the agenda at the National People's Congress. And that will be a source of relief to those on the platform, because answering any questions about which way the country's trade gap is headed, and its likely impact on the economy, would be depressing and would have to be hedged, because it is doubtful anyone knows what is really happening. The chorus of warnings about the constraints which the balance of payments will impose on China is growing by the week. The voices cover a wide range, from Hong Kong-based economists to United States-based dissidents. Of a list of 10 problems endangering China's economy, US-based Princeton China Initiative (PCI) ranks the balance of payments issue third, behind the appalling problems facing state enterprises and facing the management of the banking system. Because PCI is reckoned to be the biggest group of dissidents and exiled scholars outside China, its views will carry little weight in Beijing. But they are far from unique. Closer to home comes a similar message from Morgan Stanley, whose chief executive had an audience with Premier Li Peng earlier this month. If he gave any insight into the true state of China's balance of payments, it certainly has not filtered down to Hong Kong where analysts say they, like everyone else, are working with figures that do not add up. Using experience, best guesses and perhaps a fung shui compass, Morgan Stanley feels the Chinese authorities will have to get serious about the constraints imposed by the balance of payments. ''It is likely to be a major factor in economic policy, despite voluble media assertions to the contrary,'' one broker said. The obvious concern is that the negative flow of capital will endanger the currency, and Morgan Stanley says the present policies depend on maintaining confidence in the yuan at all costs. Luckily, the US$12.18 billion trade deficit was more than matched by an estimated $20 billion of inward foreign investment, but hard conclusions about trends are difficult because of the unreliability of the data. Figures put out by the Chinese statisticians, when measured against the official figures compiled at the other end of the trade chain, show some serious aberrations. ''The gap between the picture seen from Beijing and the one the rest of the world is receiving is quite radical,'' the broker said. Anomalies abound. December's industrial output figures showed 29.8 per cent growth in industrial output, or a month-on-month rise of 18.2 per cent - so spectacular as to be implausible, the bank says. The end-of-year swings in output, investment sales and trade could be the result of quota-filling and target-matching, it suggests. The problem could be one of description, with some exports through Hong Kong not being recorded. More serious is the practice of under-invoicing. This makes nonsense of the official figures. From Morgan Stanley's calculations, it seems that Hong Kong's re-exports from China last year exceeded its imports from China by about $11 billion. This miraculous ability to re-export from China so much more than is imported has endured for several years now. Rather than a reduction in inventory, this is more likely the effect of mainland exporters building up their Hong Kong dollar asset base, the analysts say. Looked at another way, and by using data from elsewhere, it seems that if Hong Kong's re-exports from China rose 17.5 per cent, China's exports to the rest of the world sank by seven per cent. That is not possible. The investment figures released each December are also bizarre. The 1993 figures will probably reflect the pattern seen a year earlier, when they indicated that 36 per cent of investment expenditure occurred in December. That is highly unlikely. With figures like these, some of which can be measured against known values, it is no wonder that uncertainty hangs over the rest of China's economic data - particularly inflation, which some observers now believe is climbing to the high 30 per cent mark in many areas.