Funny but you quoted merriam Webster yourself
So were you abusing a dictionary
In any case a look at variou translations leads one to the same conclusion
(MLV2017) But I wished to do nothing without your viewpoint; in order that your good deed may not be according to necessity, but according to your voluntary deed.
(ALT) But I wanted to do nothing without your consent, so that your good [deed] shall not be as by necessity but by a voluntary [action].
(Mace) but I would not do any thing without your consent; that your benefaction might not appear any ways forc'd, but perfectly voluntary.
(Douay-Rheims) D:ModulesBut without thy counsel I would do nothing: that thy good deed might not be as it were of necessity, but voluntary.
(EMTV) But I wished to do nothing without your consent, in order that your good might not be by necessity, but by being voluntary.
"Voluntary" simply means, "of the will".