Concerns

You need to have it looked at by a mechanics. The abs warning light is tripped by a number of things. Most common being faulty wheel speed sensor(s) - these are quite often damaged or interfered with by dirt/snow, etc. These are not expensive to replace. More seriously, the problem may stem from faulty abs pump/module and its replacement can cost up to $1,000! However, this fault is usually accompanied by the AST/DSC warning light. Good luck and have the brake system checked asap, you are currently driving without the benefit of abs brake!

If you are a DIY person you can pick up the ABS module online for much less than $1000. Check EAC tuning. Great knowledgea'le folks with awesome prices. Plan on a significantly large annual maintenance budget if you want to keep the car running in tip top shape. Mine certainly earned me the title "novice internet mechanic" about a month into ownership.

What year is your 540? My 1997 uses a similar Bosch module to my old 1998 Audi A4. I removed it myself and had it rebuilt by Module Master (modulemaster.com) for somewhere around $200. It comes with a 5 year warranty, thus it is absolutely pointless to waste money on a brand new one. Only thing is you're stuck without ABS for about a week - but you already are, so what's the difference?