اسلام عليكم ورحمة الله وبركاته ‎, Shaykh, may our agency generate leads for POS/payment firms whose merchants may partly or mainly sell non-halal goods/services—restaurants/grocers with alcohol, pork or non-zabiha; salons serving women (incl. non-Muslim marriages); or trucks transporting such goods?

This is too general, if you know they will deal in that which is forbidden and you are bringing them leads like a mixed gym for example or an institute that provides riba financing then this is forbidden. But if it is a halal business but maybe they will do something forbidden then we do not base rulings upon maybes we base them upon certainty which is that they are allowed.