It isn’t whether he can afford it, it is that the donation was large and real, he meant it.
Romney did not accidentally donate 3800 dollars to Christie’s political future.
Romney’s campaign manager Matt Rhoades married a Christie operative last summer (when he should have been working on Romney’s campaign and getting ORCA to work) and she will most likely work for Christie’s primary campaign. Romney knows the GOP-e have decided 2016 will be Bush-Christie and he’s falling in line so he can be a part of the campaign.