My FIL lives by himself. He's elderly but in reasonable health most of the time. He is quite well off (over 300K in the bank we think) but doesn't spend much as he prefers a quiet life and simple things.
The issue is that he asks us to get things - like DIY stuff so my OH can fix things which are broken when we visit (we don't mind doing this) and presents for the kids (he says can you put £50 in a card for them from me) - but without prompting will usually not pay us back. My OH and I have completely joint finances. I bring home twice as much as him (irrelevant really, but just for context).
He is generous at other times, e.g. he gave us £1000 each (me and OH for Xmas last year). My OH says that because of this we can't ask him to pay us back as it's too awkward. But I say that we put that cash into the mortgage so don't actually have it to give to the kids and anyway he gave it as a gift, not a fund for us to hold for him to spend when he wants/ needs to.
I find FIL really difficult. He's used to thinking only of him, so for example when we visit, it takes around 6 hours to drive to his. He doesn't get milk or bread in for us or any other food. After we arrive I have to go to the supermarket to stock up on basics. His wife died a couple of years ago and he's been trained to be inept (or lazy??) around the house so I don't expect him to cook us meals but to order an extra pint of milk and buy enough bread for us all to have breakfast is really no big ask and certainly within his capabilities. I can't figure out whether my OH is being unreasonable or whether my frustration at being expected to join my OH's family in treating my FIL like someone who really can't do anything for anyone else is clouding my thinking about this.
YABU - yes it's unreasonable to ask your FIL to reimburse you for small sums of money when he's generous with presents sometimes.
YANBU - FIL should reimburse you and if he doesn't do it without prompting it's reasonable to expect OH to prompt him.