There are a few factors involved which could make it legal. If during the time that you rented this apartment the stain occurred and you were only there for a short time, then an official quote on the re-dying or relaying of carpet in your apartment was given and this was deducted from your security deposit then yes it is possibly legal. If, however, you were in that apartment for a long period of time (~3yrs or more) or they didn't have an official quote or it wasn't written into your lease agreement then they cannot just say "you owe us $400". In the area I live (in the U.S.) after 3 years of renting a renter is no longer liable for carpeting and can request a cleaning/replacement from the landlord. Your security deposit when you sign a lease is to cover any cost of ware & tare when you leave in case you skip town and leave a mess that will require a cleaning cost. Unless there is a specific outline in a lease agreement that states that you are responsible for anything specifically in that apartment the only thing that they should be able to charge you for is repairs to the apartment charged to your deposit you then receive the remainder back. Painting and carpeting are usually considered to be the responsibility of the complex unless you were not there for a very long time and there is extensive damage done. Basically if you didn't sign something that said you would be responsible for the carpet or if they don't have receipts for an exact amount of said repair then don’t let them stick it to you!!!

It depends squarely on the agreement, as written in you lease or rental contract. if this type of damage and its subsequent repair or replacement, is in your written agreement, then yes, you would be liable and responsible for the carpets repair, cleaning or replacement.