IL- TDS RTN invoice reversed excess than amount available on std invoice TDS

@10%

Hi,

We made Prepayment invoice for Rs 50000 with @10% tax code so TDS is 5000

STD invoice with 3 lines out of 1st line with rs16000@10% tax  so TDS 1600

rest of two lines  with value Rs 4000 but no TDS attached being both are service tax & KK cess

now issue is if we apply prepayment on this STD invoice for Rs 20000 system is reversing rs 2000 instead of 1600.

is this is the indent behavior ? pl confirm

Thanks in advance

Thriveni