Generally the approach to this, using standard, is to move the stock you want to be used for a specific purpose to a different location, unless you mean a strict jobbing scenario but this is then items purchased/made for a specific reason which is different to quantities as you describe.
If you are using Warehouse Management, I would highly recommend reading Microsoft Business Solutions (MBS) Training Material 8387A: Navision 4.0 Warehouse Management. This book should be available through either PartnerSource, CustomerSource or through your consultant. the book is a fairly quick read, you can feasibly read through the entire material in one to two days.
For your immediate query, I would primarily focus on chapter 3.
Chapter 3 will teach you how to use the blocking feature of bins. Note that this is also available at the Zone level. (On the Navigation Pane, go to Warehouse > Setup > Locations, then click Locations > Bins. Look at the Blocking field)
Chapter 3 will also teach you about internal warehouse processes. You will want to pay special attention to internal picks and put-aways, which gives an example where a salesperson desires an item for demonstration purposes, which they return later. Extrapolating, you could create virtual (read: non-physical) bins for each of your Salespeople; and when they need an item allocated to them, they have an internal pick that transfers the item to their bin. As a last precaution, I would make sure you understand bin types so that the bins are set up for put and pick only.
If you are not running WMS, I think you can still use blocking features. There’s also setting things to reserved, but I haven’t used the functionality much at this point.