[Miguel]

Any number multiplied by "-1" will change its sign:

pawn Code:

new value = strval(inputtext);
if(value > 0) GivePlayerMoney(playerid, value * (-1));
else
{
    // The value was negative or just zero.
}

Or:

pawn Code:

new value = strval(inputtext);
if(value <= 0) value *= -1;
GivePlayerMoney(playerid, value);