Bismihi ta'ala

Khums must be paid in its due time, without delay, and one must fulfil what is in their dhimmah, so as to not fall in sin and to make what they purchase from that due money halal and not usurped. 

The solution to this, if someone is not able to pay the complete amount or any amount in its due time is to do what we call a Muṣālaḥa.

A Muṣālaḥa is a mutual agreement and solution agreement between the mukallaf and the Marja' taqleed or his wakeel. 

What happens is the mukallaf speaks to his/her Marja' or his wakeel, and explains that they cannot pay their due khums, and it is transferred in a technical way from khums to a long-term or short-term loan. The mukallaf will then be responsible to honour the loan and it is to be given whenever they are able to.

By doing this, the mukallaf will be free of khums payment until that time, and their money will be halal.

So, best thing to do is to speak to the local wakeel of your Marja'.

And Allah knows best