It is an important distinction that workers do not sell their labor, they trade their labor power for a wage.

If one sold their labor, I.E. that things they themselves create this would draw a distinction that the worker can withold the labor (I.E., the object or service created) to negotiate a higher price.

But this is not the case, a wage laborer is expected to surrender all of their labor created within a certain timeframe to their capitalidt for an agreed upon price. This can vary widely and is a huge aspect of why people end up being exploited. This is because the capitalist can set the scheme to expect only a small amount of labor up front, while gradually cranking up the labor poer demanded for stagnated wages.