Is there a single adjective for "mercenary-like attitude"?

The word mercenary is itself an adjective as well as a noun:

mercenary [mur-suh-ner-ee]

adjective

1.working or acting merely for money or other reward; venal.

2.hired to serve in a foreign army, guerrilla organization, etc.

(From dictionary.com)

"I would ask my best friend to help us in this project, but I don't trust him. He's not a bad guy, just very mercenary."

If you don't want to use "mercenary" as an adjective, consider "venal". Mercenaries are bought. They have a reputation for fighting for the highest bidder despite personal convictions (if they have any).

venal
adj
1. easily bribed or corrupted; mercenary: a venal magistrate.
2. characterized by corruption: a venal civilization.
3. open to purchase, esp by bribery: a venal contract.
from Latin vēnālis, from vēnum sale
Collins English Dictionary

I see the word as being akin to "buyable"

Unscrupulous.

un·scru·pu·lous

ənˈskro͞opyələs/

adjective: unscrupulous

1. having or showing no moral principles; not honest or fair.

As in:

Last month, several unscrupulous stockbrokers used insider knowledge to make gigantic personal trades.

Source: https://wordsinasentence.com/unscrupulous-in-a-sentence/

According to Merriam-Webster, something is meretricious if it has the nature of prostitution, is tawdrily and falsely attractive, or is superficially significant. The authors there also note that it was their word-of-the-day on February 11, 2013, and you can still hear the podcast from their website. In the podcast, someone uses the word to describe music intended to appeal to the audience although not really very good, and other uses echo this sense of motivation by money where that motivation is not quite respectable.

A little look around Google Books suggests that the word is mostly used in legal contexts or in style manuals discussing the word itself. I did turn up one more literary use of the word, in William Faulkner’s Light in August: "He watches quietly the puny, unhorsed figure moving with the precarious and meretricious cleverness of animals balanced on their hinder legs; that cleverness of which man animal is so fatuously proud and which constantly betrays him by means of natural laws like gravity." (I only found this passage cited in other books; perhaps the full text of Light in August is not yet available online.)

The word is also in Johnson’s dictionary, but of course that takes up back a while.

In The History Boys: A Play By Alan Bennett, a character uses the word and then must explain what he meant by it.