再添几句，因为是刚性的贸易，国内因为本币贬值而获益，国外因本币贬值而受害，这也是为什么很多国家采用本币贬值来增加国内产品竞争力，并使本国获益的原因了，不过相对来说本币贬值造成的影响也是双面的，那就是国内的通货膨胀。

Since the question is written in English, i find it appropriate to reply in English :)
Below is what i think:

The question has clearly stated that both the demand for imports and exports are inelastic, hence, i would like to look at it as the demand (in quantity) will not be affected by the fluctuation in the foreign exchange.

For illustration purpose, say the domestic currency is RMB while the foreign currency is USD, and the exchange rate is currently USDRMB 6.00 and after it is depreciated it would be USDRMB 6.50.

Assuming that the demand for exports is 50 units priced at RMB 100 per unit, while imports is 80 units, also priced at RMB100.

Domestic Trade Balance:
Day 1: total exports = RMB100* 50 = RMB500; imports = RMB100 * 80 = RMB800, trade deficit = RMB300

After depreciation: total exports = RMB100* 50 = RMB500; imports = RMB100 * 80 = RMB800, trade deficit : RMB300

Foreign country's trade balance:
Day 1: total exports = RMB100* 80 / 6.0 = USD133.3 ; imports = RMB100 * 50 /6 = USD83.3, trade surplus : USD50

After depreciation: total exports = RMB100* 80 / 6.5 = USD123.1 ; imports = RMB100 * 50 /6 = USD76.9, trade surplus : USD46.2

Since the contracts are all denominated in DOMESTIC currency, the balance of payment is not affected by the movement in the exchange and the Demand (quantity of goods) also is independent (inelastic) of price, we can hence conclude that:

1) the domestic trade balance (in RMB) will not be affected
2) the foreign country's trade balance will be affected because the contracts are not fixed in its currency (the impact however depends on whether the foreign country's exports > imports or another way round)

在需求无弹性的条件下，会产生贸易逆差，而不是贸易顺差。