It probably says fees, rather than dues.
The fees for "occasional offices" (such as the celebration of Holy Matrimony) were an important part of the Vicar's income.
If the couple marry somewhere else instead, then the vicar of that place might be accused of "poaching" a fellow clergy-man's fees. Essentially, therefore, it boils down to a question of professional ethics among clergyman - you don't shear the sheep of another parson's flock without his consent, and that consent will only generally be forthcoming if he gets paid his fee regardless.
In your case, I think we must assume an established custom that the vicar of Midrim had a right to fees for Panton marriages - perhaps because the parish of Panton had been carved out of the parish of Midrim (or had at one time been a peculiar of Midrim or some such arrangement) and that the fees were reserved to him when it was created in order to protect his income. I am not familiar with the history of these parishes, so cannot assist further on this point.
That having been said, the issue was a controversial one and by the end of the 18th century the demand for a fee by the bride's parish priest when he did not celebrate the marriage had become all but unsustainable. This is what the 6th edition of Burn's Ecclesiastical Law (1797) has to say on the matter (volume 2, pages 481 - 2):
Mr Johnson says, it was an ancient custom, that marriage should be performed in no other church but that to which the woman belonged as a parishioner; and therefore to this day, the ecclesiastical law allows a fee due to the curate of that church, whether she be married there or not. And this fee was expressly reserved for him by the words of the licence, according to the old form, which is not yet disused in all dioceses. But it is said, that judgment hath been otherwise given in the temporal courts. Johns. 188, 189.
So in the case of Thompson and Davenport, M. 13 W. The plaintiff libelled against the defendant, setting forth a custom in the parish of Ellington in Derbyshire, that of every woman who is a parishioner, and dwelleth there, and marriage with a licence, the husband at the time of the marriage, or soon after, shall pay to the vicar 5s as an accustomed fee; and so brought his case within that custom; the defendant suggested for a prohibition, that all customs are triable at common law, and that the plaintiff had libelled against him, setting forth the customs as aforesaid. And a prohibition was granted. Lutw. 1059.
And in a late case, upon an appeal to the arches, Sir George Lee not only declared against the custom as an unreasonable custom, but gave costs against the clergyman with some warmth.
And Sir William Blackstone says, of common right no fee is due to the minister for performing such like branches of his duty, and it can only be supposed by special custom; but no custom can support the demand of a fee without performing them at all. 3 Black 90
There is, of course, a lot of technical legal stuff in that passage; but even without any detailed of canon law and ecclesiastical jurisdiction, it should be fairly obvious that the gist of the message is "these fees used to be payable, and some people still seem to think they are - but the courts are pretty adamant that they are not". Sir George Lee was Dean of the Arches; and the Court of Arches is the appeal court of the Province of Canterbury and was, therefore, at the time in question the highest ecclesiastical court in the land. If even the Dean of the Arches was giving judgment against the payment of such fees, then we may take it that they were, by 1797, essentially unenforceable.
You do not say what date this was - but i am assuming it is probably mid-18th century. Am I right?