Guest asking refund due to recalc of fees

[Annie-and-TJ0]

Hello, I have guests that wanted to alter their reservation from 49 nights to 27 nights due  to work.  I totally understand it and agreed to help them alter it and made sure that their will be no penalties added to both parties. No one realized that the 29% discount will be taken out and none of us knows that it will do a recalc according to original pricing (without discount). Should i refund them the 29%discount because they are asking for it?  They said that they are willing to pay for the extra night for the discount to be applied.  Please help.