What would you do if...

So my boyfriends birthday is next Monday and for his birthday I got him an acoustic-electric guitar that he would always tell me about. Well, the guitar was ordered back in March so I’ve had it hidden since then. I asked his parents and my mom if they would be interested in splitting the cost with me, both agreed.

Well, long story short: the closer it gets to his birthday the more his parents try to take full responsibility for the gift, setting a time to give it to him (I’m giving it to him Saturday so he can play it on Sunday - he plays in our church praise band) but his parents seem to have this idea that this is coming from them alone and how they’re so awesome for getting him a killer gift and how he’s going to be so surprised and THEY can’t wait to give it to him...

It may be a little ridiculous to be getting mildly upset over it, but I only asked if they’d be interested in splitting the cost to save money (I’m in grad school so I’m not able to work full time)...but we’ve been together for over 3 years and I’ve worked so hard to be able to finally get him this guitar so I’m a little upset that his parents are trying to take the credit when the had no idea that the guitar even existed or that he wanted it until I mentioned splitting the cost...which I see now was a terrible mistake on my part. 🤦🏼‍♀️

What would you do? I’m stuck in the middle of telling them how I feel and risk coming across as rude and being respectful and just not saying anything at all.

Side note: my boyfriend and I live together so I’m having to keep the guitar at his parents house until I give it to him.