Praise be to Allaah.
Firstly:
What the agent did of
buying the land for cash then selling it to you for instalments, is a valid
translation and there is nothing wrong with it. It is not a condition of
validity that the sale be recorded or written down; rather if the proposal
and acceptance are done, and the owner gives up the land and the agent takes
it then sells it to you, then he has sold what he owns, praise be to
Allaah.
In this case the agent may
sell the land for a price by instalments that is higher than the price for
cash, but he has done well by not charging you more.
Secondly:
It is haraam to charge tax
(mukoos) on land and other things, and it is prescribed to avoid them by all
permissible means that will not result in greater evil.
Your purchase of the land
is valid and there is no sin on you in sha Allaah.
See also the answer to
questions no. 82111 and
39461.
And Allaah knows best.