Meaning of a sentence in A History of Britain about Edward I

The word pawned means that he got a loan, leaving some valuable objects as collateral. If he did not repay the loan within a certain amount of time then the pawnbroker could keep the items.

Redeeming, in this context, means the act of repaying the loan plus any interest and recovering his collateral.

The highlighted passage means that he entered the temple, promising that he was about to redeem the loan, but then took the Templars' money by force.

And saying that he "staged" a robbery just means that he robbed them. It doesn't indicate that he made any false performance, or claimed that other people had robbed them.