Hi ,
Please find here with answer for your issue you mentioned .
Normally the Valuation is different than the GR/GI you do . In terms of the quantity you can issue the First in quantity as First out .
But the valuation of the quantities depends upon other factors
Normally a material will be valuated with either S or V during its creation in its accounting view with a value / price .
Say suppose X qty is taken with a Y price with a PO and the total value of the stock in is X.Y during the GR.If the material is valuated at S then the diffrence in the PO value and std price will post to the stock account and if valuated at V the diff will be posted to a price difference account . Even though if you have a differentiated IN value for the material based upon FIFO , you can not issue the material with that diferentiated price .
Please check the below link for more details on valuation .
http://help.sap.com/saphelp_46c/helpdata/EN/ff/515fab49d811d182b80000e829fbfe/frameset.htm
Regards