I want to take the 'amount' field in Vendor receipt form out of account.voucher and use it in my own custom model.

I want to subtract the value of 'amount' from a float variable called 'returns' present in the model named 'record'. But I'm unable to figure out how to take 'amount' in a variable in my model after the "validate" button in receipt (in account.voucher) has been pressed.