Problems filing an FIR and queries on fraud case and cheque bounce case

Dear Sir/Madam, I had lent an acquaintance of mine some money to be returned with interest within the stipulated amount of time. But the person declined to pay citing reasons like loss in business where as I know he owns properties and his current business is doing good. Once we figured that he is not willing to pay us in good terms, we made him to do a promissory note done with amount and times for payment mentioned. Also I have taken three cheques for security. On looking at the signature on the cheques given and the agreement, it looked to be forged. Further I also figured that the details presented to make the agreement are not correct like address. I consulted a lawyer and he adviced me to file a Fraud case immediately as the accused is trying to vacate his house before the cheque dates. Lawyer drafted a complaint listing the sequence of events and following sections were said to be applicable for the offence he had done (120-B, 406, 420, 465, 468, 471) But to file an FIR, cops are now asking me to return the original cheques as they need to send it to FSL for signature verification. If i give them the originals now I will not have anything with me to prove that accused was a fraud and will not have a cheque to present to file a case under section 138. I request the IO to file case under 420 and 120-B if not all sections, but they are demanding originals back irrespective of the sections. I suspect he might try to abscond by the cheque date. Help me what I can do further so that FIR is documented by the cops immediately or should I wait for the cheque bounce to file both fraud and cheque bounce case together. Any suggestion is appreciated. Regards,