You can use fields of type related to update fields which are not part of the actual model.

Here is an example from model stock.move:

'origin': fields.related('picking_id','origin',type='char', size=64, relation="stock.picking", string="Source", store=True),

This code maps the origin of the stock.picking to the origin of the stock.move. So you can change the origin field of stock.picking from within the stock.move model.

Update:

You can also use a function field and define a method for argument fnct_inv. (See the Developer documentation)