Allow negative expense amounts

Allow entering negative expense amounts. This would be helpful when recording refunds or adjustments to previous expenses, without deleting or adjusting the previous expense, which may already have been settled.

Also, in some cases the refund amounts are not exactly the same as the original amount, for example when an expense was made in a foreign currency the refund amount may be less or more depending on rate fluctuations. Or the refund may be smaller because some portion of the amount was non-refundable.

Being able to enter an expense with a negative amount would make it easy to handle these situations.