Upvote:4
Mali is one of the 20 poorest countries in a World with extremely low developed governmental infrastructures.
In such a cases officials rely on stamps as a proof of the legitimacy of a document, not a secured database.
Your visa clearly states Length of stay: 3 months, so do not worry and in case you'll be asked by a board guard, just tell them it was an embassy clerk, who made this correction.
Upvote:4
Nobody can tell you definitively if you will face problems however I would not fret over it. Some of the developing economies in Africa are more lax on visa protocol and the scenario you describe is not unheard of. I know because I am originally from there. The unedited 3 months length of stay written on there is in your favor.
If you're that concerned you can get a letter from the embassy. Worst case scenario immigration officer at the airport asks for some payola