California-based Prime Healthcare Services won approval earlier this week to buy Woonsocket’s Landmark Medical Center. But the approval came with conditions. One is that the company keep Rhode Island regulators abreast of a federal investigation it’s under for allegedly over-billing Medicare. Another is that Prime invest in primary care and in preventing hospital re-admissions. The Department of Health expressed some concern about Prime’s practices at other hospitals, but department head Michael Fine said the terms of the deal to acquire Landmark should allay those concerns.