You can't force people to be generous.
It is correct that when giving of one's own possessions, the giving should be done voluntarily and not "under compulsion" (2 Cor 9:7). But the gospel is not one's own possession, nor is any ministry done in the service of God.
If you are employed in a modern company, anything you create is owned by the company and not by you. So if you create a resource while at work, it belongs to your employer for the benefit of the whole company. This was the same in Jesus' day too. If you were hired to work in a vineyard, the produce of that vineyard belongs to the vineyard owner and not to you. Workers would be granted some of the fruits their labor produced, but that provision comes from their employer and not their ownership of it (1 Cor 9:7).
So it must be asked, who are we ultimately employed by? Anyone serving God is subject to him and anything they create while in his service belongs to him, for the benefit of his kingdom.
Which of you whose servant comes in from plowing or shepherding in the field will say to him, ‘Come at once and sit down to eat’? Instead, won’t he tell him, ‘Prepare my meal and dress yourself to serve me while I eat and drink; and afterward you may eat and drink’? Does he thank the servant because he did what he was told? So you also, when you have done everything commanded of you, should say, ‘We are unworthy servants; we have only done our duty.’ ”
While giving of one's own possessions is a matter of generosity, the free giving of ministry is primarily about obedience. After freely giving ministry we should not claim to be generous, rather "we have only done what was our duty".