out of which for 80 quantities, inbound invoice (No Goods receipt) is created and for 20 no inbound invoice is created yet
Your above statement is contracdict to each other. Nevertheless, if your client have inwarded 80 nos., and do MIGO and transferred to Unrestricted, then system would consider this. Of course, there is a field "GR Processing Time" in MRP view of material master which is nothing but the number of days for inwarding, depends on which also, stock availability would vary.
G. Lakshmipathi