PO with material group and GR with specific material

I have a business scenario that we have general materials like bolts, bearings and other engineering materials for plants. There are 3000 materials like this. Now we are sourcing this to a shopkeeper whom we will give a PO and he will be responsible to deliver it on an agreed price.

These are daily based items so logically speaking we cannot give PO for each material.

Is there a possibility that I give an open PO to my vendor with a good value. At the time of GR, I receive specific material so that I can track what has been received so far.

I tried using material group by giving Acct assignment 'K' , short text and material group but it didnt work.

Any other solution?

Regards,