Can Tenant Choose Where Money Is Applied

[Dhaval Shaw]

Hello,

My tenant has an outstanding expense for plumbing for which they owe me $300.  This bill is over a month late.

Today they paid a some money and claimed it is for rent.  I told them that those funds will have to be used for the plumbing expense they owe me since it is an older charge.

Can they dictate where the money is used?  If so, what prevents them from continuing to pay the rent and never paying the plumbing charge?

Thanks in advance.