Salesman giving a discount

Q: Some salesmen don't accept small change when we make a purchase. Example, I purchased a water bottle for 11 AED and I had 10 AED with me. I gave it to him and said that I will give the 1 AED later. He said ok and the next time when I gave him he said, "Its ok leave it". He is not the owner of the shop. He told me that since its a large industry, 1 AED doesn't matter.

In the college library there is a rule that if a person failed to return a book within a certain time limit, he had to pay a fine of 1 Rupee per day till the book was returned. I returned the book 2 days late and told them that I am late. They told me not to repeat it next time. They did not take the 2 Rupees from me. It was a librarian and not the owner of the college or library who told me that.

Online advertising

Q: Please clarify the following for me:

Generally, companies run through advertising and it thus plays a major part of many businesses. Therefore:

1. Some people specialise in doing the marketing/advertising for the company and get paid for their services.

2. Some people with computer abilities upload such ads on websites and get paid by the companies.

Are these types of jobs impermissible? If so, will magazines, radio stations etc. charging people for advertising also be impermissible?