make-weight

I don't think this is right. 'contrapès' = counterweight.

A make-weight is 'something put on a scale to make up a required weight' (Collins). So, for example, if you wanted to buy exactly 200gm of liqueur chocolates, but a number of whole chocolates came up to only 194gm (and another whole chocolate would bring the total to 212gm), the chocolatier might put on a makeweight of a small piece of chocolate to make a total 200gm exactly. In the pre-electronic era salespersons often did such a thing because the price of an exact amount like 200gm was easy to calculate, whereas the price of 194gm would have to be worked out with pencil and paper.

So what we are looking for is:

torna (GDLC)
2 Porció que el venedor afegeix a una mercaderia perquè acabi de fer el pes demanat pel comprador. Donar un llonguet per torna.

Having just done a Google search for 'makeweight', this would appear to be a term which crops up a lot in sports. Any ideas what the Catalan might be in this sense?

examples:
It is believed that Finnan is a makeweight in the deal that will bring £10m winger Albert Riera to Anfield from the Estadi Olímpic Lluís Companys

The Daily Star says United offered Giggs as a makeweight in their attempt to sign Owen from Real Madrid

Papua New Guinea, the supposed makeweights of the 'super pool', threatened to pull off one of the biggest shocks in World Cup history when they scored three tries in a 21-minute spell to lead 16-12 at half-time.