how to make partial payment first by partial payment and the by residual item

hi,

I have posted an invoice with value 1000 then partial paid 200 using paritial payment and now i want to post another payment of 200 using residual item but now system disallows me and when i configured the payment difference account, system is adjusting 1000 rs against vendor and posting 200 for vendor in separate couple of line.Why is it happening

cash a/c 200 vendor 200- payment diff a/c 1000 vendor 1000-

Regards

Khagesh