This is easily dealt with with the right cleaning materials. Ideally, you want nothing other than a damp cloth to rub away the dirt and grime that resides on the material. Minimal water and minimal effort will produce the best result. You don’t want to wear down the surface with vigorous rubbing. You also don’t want to damage it with soap and other chemicals. As for deeper stains, it is best to refer to manufacturers care instructions and use a specialized conditioning treatment. This is the only way to remove the stain without damaging the dyes.
Then there are the issues of excessive moisture. Too much water can damage and misshape the wallet with irreparable damage. If the leather does become wet, through dampness or spill, be careful when drying it out. The first instinct may be to blast it with heat from a hair dryer to remove the water faster. This may actually damage the wallet further. Speed drying method can manipulate the chemical structure of leather, with horrible effects. Take it slow and try and maintain the right shape as it drys. Room temperature and slow processes will make all the difference.