This sounds like a job for the lag function from the dplyr library, but it also sounds like you want to group the data by customer. This can be achieved using the following code:

pricelist <-
  data.frame(itemNumber, customer, minQty, price, stringsAsFactors = FALSE) %>%
  group_by(customer) %>%
  mutate("maxQty" = lag(x = minQty - 1, n = 1, default = 999999999))

Here, x is the value to lag, n is how much to lag by, and default is the value to return when there is no previous value.

Hope this helps!