During an e-connect integration, we might come across a situation where the currency ID remains blank in the transaction entry despite having a functional currency set up correctly. The issue can be resolved by making sure there is a checkbook ID in the company that has a currency ID associated to it or an item associated to a currency ID.
Another place to check is to ensure that the functional currency Index value in company database (MC40000) matches the currency index value in Dynamics Database (MC40200).
Source: Dynamics GP Forum Help Thread
