3 year commision payment for a 3 year tenancy agreement!

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I have accepted/signed a commission agreement for a 3 year tenancy whith a fairly large agency and therefore didn't check the small print. The entire commission is, however, not spread over the 3 years, but collectable over 6 months. On top of that I was initially requested to stem the Deposit, even though the tenant had paid the agency the deposit.
I simply didn't pay (agency had deposit and 2 month rent collected) and have started to collect the rent myself. I have a good relation ship with the tenant and this has been going on for now 5 months.

I am willing to pay, but not the entire amount over 6 months. The agency has in the meantime registered the deposit and offered to spread the commission over 10 months, but not further than that. They now threaten to go to court. What do you recommend? I am willing to pay 1 year now and each year the same amount again (3x12 months), but that is not what I signed.