Institution refuses reimbursement for grant awarded?
What you need to do is work it out locally. If necessary, the grant funder may need to make a judgement. But it seems like you were trying to get around a restriction by somewhat questionable means. Not necessarily ethically dubious means, but questionable. A pedantic interpretation of the statement in the grant, and ignoring its likely intent is what got you into the current situation. You need to argue that your interpretation is correct and you need to get others, who have some authority here, to agree with you. 

I can't guess whether you will win this or not, but the situation is between you, the institution and the funding authority. Make your best argument there. And note that you can't impose your own will on this. 

If this is PC level equipment, I doubt that it is going to be a serious setback to your finances. Maybe it is best to just absorb the cost and spend the funds on things that aren't so close to the margin of acceptability.