keep first row after calculating difference between rows with dplyr::lag

Question

My question is similar to this OP and this OP, with a minor difference that seems to be overly complicated.

Example of my data:

ind_id   wt   date
1002     25   1987-07-27
1002     15   1988-05-05
2340     30   1987-03-18
2340     52   1989-08-15

I am calculating the difference between wt values after group_by(ind_id).

To do this:

df<-df %>% 
    group_by(ind_id) %>%
    mutate(mass_diff=(wt-lag(wt)) 

This gives me this output:

ind_id   wt   date        mass_diff
1002     15   1988-05-05  -10
2340     52   1989-08-15  22

But, the output I want should keep the first wt record, not the last.

Desired output:

ind_id   wt   date        mass_diff
1002     25   1988-05-05  -10
2340     30   1989-08-15  22

Note that the wt column is the only one I'd like to have maintained from the first row. (Keep in mind that this example is overly simplified and I am actually working with 18 rows).

Any suggestions (using dplyr) would be appreciated!