As a Project Manager, one of the most important abilities that you need is the ability to influence others. For instance, if you need the best programmer for one of the modules of your project, you need to convince the functional manager that your project is the most important one on the floor.
This is why interpersonal and team skills count as one of the major tools and techniques of the Acquire Resources process.

It should be noted that this is not limited to negotiation. It also includes managing conflicts and motivating. However, you may have to negotiate with the following,
Functional Managers
As cited in the example above, you may also have to negotiate for the timeframe the resources are assigned as well as their ability (if they work on multiple projects)
Other Project Managers
We just talked about how resources might be assigned to multiple projects. You may have to negotiate with the Project Managers of those projects when it comes to sharing resources
Suppliers
When we talk about negotiating with suppliers, an additional layer of policies, guidelines, legal and other criteria should also be considered
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