IF statement based on dropdown result in google sheet

I'm working with google sheet and seeking some help.

Below is the situation and expected outcome.

- there is Income column, Payment column, Currency column, Balance column

• Income and payment is where numbers are put

• USD, JPY, THB, SGD in dropdown for currency

• wrote below to detect which currency is selected =IF(H7="THB",G7*, IF(H7="USD",G7*, IF(H7 = "JPY",G7))) (above calculation is not done in any of the 4 columns mentioned above)

• Balance column is using below calculation =IF(AND(F7="",G7=""),"",I6+F7-G7)

1: Input number of income / payment 2: Currency selected 3: calculate the currency rate 4: reflect the result in THB and perform the existing calculation on balance column

Could anyone please teach me how to do this.

Essentially, I would like to use the result of calculation performed based of pulldown result to perform another calculation.

In other words, in php or any language, I want to assign the calculation result to a valuable then use that valuable in different argument.

Thanks !