What a person expects to be paid is anchored to three things, and only one of them is the job they are doing for you.
The three anchors
The local market for the role. Published ranges exist and are reasonably well known inside the industry. A role paid below the range attracts a narrower field and loses people faster.
What they were paid before. Movements between employers here usually involve an increase, and somebody who took a lateral move to join is aware of it.
What the person at the next desk earns. The most powerful of the three and the one clients never see. People discuss pay, in every country and in every office, and a person doing more skilled work than a colleague for less money finds out.
The annual increase
Expected. Not necessarily large, and expected. A year with no movement reads as a signal about how the person is regarded, whatever the reason for it.
Performance-linked increases are less established here than in some Western markets and are becoming more common. Whether your provider operates them, and whether you have any input, is worth knowing before the first review rather than during it.
What you can and cannot do
Under staff leasing you are not the employer and may not see the salary. Some providers disclose it, some do not, and both positions are defensible.
What you can always do is tell the provider that you want somebody retained and ask what that takes. What you cannot do is negotiate directly with the person, which puts them in an impossible position and may cause a problem with their actual employer.
Establish it early
Before you need it: can I see the salary, can I fund an increase, and what is the process. Asking during a retention conversation is asking too late.
The counter-offer
Somebody resigns, a counter-offer follows, they stay. The record of this arrangement is poor almost everywhere: a substantial share leave within a year anyway, because the reason for looking was rarely only money.
It is not never right. It is right less often than it feels in the week somebody resigns, and the better question is what they were looking for rather than what it would cost to stop them.
What money does not fix
A role with no variety. A manager who does not respond. Work whose purpose is invisible. Being the only person on an account with nobody to talk to about it.
Each of those produces departures at any salary, and each is fixable by a client at no cost, which is the argument the entry on attrition makes at length.
The equity question inside a team
Where you have several people, differences in pay between them will be known and should be explicable. Somebody who has taken on more, trained others and owns a process should be paid more than somebody who has not, and where that is not the case the team draws its own conclusions.
You may not control it and you can raise it, and a provider who cannot explain a difference has a problem worth surfacing.
Talking about money with the person
Do not, beyond acknowledging that you value them and that pay is between them and their employer. It sounds evasive and it is the correct position: an arrangement where a client discusses salary directly with somebody employed by a third party goes wrong in several directions at once.
Say it plainly rather than deflecting, and then act on it with the provider.
Benchmarks
Salary surveys for this market exist and are sold, and providers generally have them. Ask where in the range your roles sit. A provider comfortable answering is one whose pricing you can reason about.