Thanks for all the feedback, questions and advice.
Many of you have understood the real situation, ie: he has a large, expensive collection already that could easily clear our debts.
The instrument he wants is not related to his mum at all and is one he has just always wanted.
If we didn't have debt I wouldn't care, but we both find the debt stressful and bad for our marriage, so I wronged assumed he would prioritise.
He is a hoarder, his entire family are, so there are many momentoes.
The debt is fairly small, however, it still bothers me. Some people may not care about a few thousand pounds, I do.
He is reckless with our finances and try as I might, he won't help look after them. It is exhausting being the person who carries this weight.
Emotions are high, but I have not daid anything g to him about this. I genuinely have supported him through everything, i just needed somewhere to vent.
I'm a planner, so yes, I've discussed possible inheritance. I may not get any, but the same thoughts apply when my parents gift sums of money, which occasionally happens.
Again, thank you all for your input. It's made me really consider our daily life.