The French midfielder arrived at St Andrew's on Thursday having been without a club since leaving Championship rivals Nottingham Forest at the end of last season.
And, rather than being paid for his short-term stay in the West Midlands, Moussi will donate the money to four charitable causes.
The organisations set to benefit from Moussi's stay at Birmingham are the 2011 League Cup winners' disabled supporters club, Stop Ebola, a church in Paris, and TEV Soma Charity, which raises money for those affected by the Soma mine disaster in Turkey.
"We knew Guy was out of contract and we invited him over to train for a week and he did quite well," said Birmingham boss Gary Rowett. "We then got him over for another week just to make sure. It was a tough week [of training] and he got through all of that.
"He's a big strong defensive midfielder and we feel that's something we haven't got so we've agreed a short-term deal to bring him in and that's reflected in what we’ve offered financially.
"But it’s an opportunity for him to come and look at us and us to have a look at him and see if he can earn something."