Microsoft is offering customers of Salesforce.com Inc thousands of dollars in rebates in a bid to woo them to its online customer relationship management (CRM) package.

Salesforce.com has long been a critic of using the installed software, which is Microsoft’s stock trade. This time, Microsoft has countered its rival by taking many of its services online and offered alternatives to Salesforce.com's products using a combination of installed software and online access to databases.

An advertisement placed in the U.S. West Coast edition of the Wall Street Journal on Monday by Microsoft said it is offering companies its products at $200 per user, if they switch from its rivals. For some companies, the incentive could add up to tens of thousands of dollars in rebate.

At Microsoft, we do not believe you should be forced to pay a premium to achieve business success, Microsoft executive Michael Park wrote in an open letter to Salesforce.com customers. The letter was part of the ad published in the Journal on Monday.

The ad was timed just ahead of Salesforce.com's Dreamforce 2010 conference in San Francisco this week. The rebate offer will be available until June 30 next year for customers who switch from Salesforce to Microsoft Dynamics CRM.

Microsoft aims to convince attendees that its customer relationship management solution offers more value, said Jamie Fiorda, group product marketing manager for Microsoft Dynamics CRM, in a statement.

But Salesforce.com chief executive Marc Benioff, a vociferous critic of Microsoft, brushed aside the software giant as a technology monopolizer and patent troll.