You didn't mention which version of SEUM you have but if you have SEUM 1.5, you can try to use "XY Capture mode". In this mode SEUM simulates physical key presses and it may help in this case. Record beginning of the transaction as usual but before you enter amount, turn-on "XY capture mode" and enter amount, then turn XY mode off and continue as usual. If this doesn't help, you may try to record whole transaction with "XY capture mode" turned on.
Just keep in mind that transactions with XY mode can be played back only on players installed on Windows Vista or newer.