“And he went out about the third hour, and saw others standing in the marketplace idle, and to them he said, ‘You also go into the vineyard, and whatever is right I will give you.' And they went their way.”
Presumably the work was falling behind with the result that His manager informed Him that more workers were needed. Or perhaps we are to see in it simply the goodness of heart of the estate owner although in that case why not hire all at once? But the purpose of the details is not in order to explain the estate owner's reasons but in order to get over the idea of a gradually ongoing situation. So He again goes out to look for labourers, this time at roughly 9:00 am. And in the marketplace He finds that there are still many labourers who have not found work. So He again selects out some workers. They would have been there from early morning, but no one had previously hired them (Matthew 20:7). To these He promises that He will pay ‘whatever is right'. To this they agree, for they know that they cannot expect a full denarius, and they are desperate to get work. And like the others they go to work in His vineyard. Note the deliberate emphasis on the fact that they are to trust the estate manager to do what is right.