Rather pay immediately via challan and claim refund later, otherwise its a lengthy procedure , they will have to issue notice u/s 245 for refund adjustment and then again you will be given some time (30 days) to pay the arrears and if you dont pay in that time and if no further time is provided to you, then the refund is adjusted. This whole thing can take more than a month.
No more interest, but if you do not pay within time limit you may be treated as assessee in default for which there is penalty. Penalty is levied by AO and not paid voluntarily. Pay immediately.
S.220(4) If the amount is not paid within the time limited under sub-section (1) or extended under sub-section (3), as the case may be, at the place and to the person mentioned in the said notice the assessee shall be deemed to be in default.
221. (1) When an assessee is in default or is deemed to be in default in making a payment of tax, he shall, in addition to the amount of the arrears and the amount of interest payable under sub-section (2) of section 220, be liable, by way of penalty, to pay such amount as the Assessing Officer may direct, and in the case of a continuing default, such further amount or amounts as the Assessing Officer may, from time to time, direct, so, however, that the total amount of penalty does not exceed the amount of tax in arrears :
Provided that before levying any such penalty, the assessee shall be given a reasonable opportunity of being heard :