Charlie Sheen on Lohan

Charlie Sheen on Lohan,  Don't ever say Charlie Sheen's not the charitable type -- because he recently cut Lindsay Lohan a $100,000 check to cover the actress' six-figure tax bill ... all out of the goodness of his heart ... sources tell TMZ.

Sources close to Lindsay tell us she and Charlie became close pals while on the set of "Scary Movie 5" back in September -- spending a lot of time together both on and off the set......ohnotheydidnt.livejournal.

During their bonding period, we're told Lindsay and Charlie talked about everything -- and at one point, Lindsay mentioned her ongoing tax problems.

We're told Charlie offered to cut her a check then and there to get the IRS off her tail, but Lindsay refused.

Fast forward to last week -- sources close to Lindsay tell TMZ, Lindsay's biz manager received a check from Charlie for $100,000.

We're told Lindsay was blown away by Charlie's generosity -- and immediately applied the money to her outstanding tax bill.

TMZ broke the story ... Lindsay allegedly owed Uncle Sam $233,904 in unpaid taxes for 2009 and 2010 -- but thanks to Charlie, that number's nearly been cut in half.

As we reported, Lindsay's slated to earn over $2 MILLION by the end of this year -- but who knows where that's going.