Bottom part of the Hourglass

Clean, 305 289 bytes

import StdEnv
@[h:t]|all((==)9)h|t>[]=[h: @t]=[h]
#s=length h/2
#m=hd(sort h)+1
=[updateAt(hd[e\\e<-flatten[[s+i,s-i]\\i<-[0..s]]|h!!e<m])m h:t]
$a b c=flatlines(map(cjustify(a+1))[fromString(fromInt(b-c)),['-']:reverse(map(map(toChar o(+)48))((iterate@[repeatn(a-r)0\\r<-[0,2..a]])!!c))])

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05AB1E, 68 63 59 57 56 bytes

IÅÉÅ9[DOO²Q#ćD_Piˆëć<¸«¸ì]ćā<ΣÉ}2äćR¸ì˜è¸ì¯ìJDSOIα'-‚ì.C

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Explanation

IÅÉÅ9
We initialize the stack with a list of list of 9's.
Each list represents a row so the length of each list is odd and the length of the last list is equal to the first input.
An input of 5 would result in [[9], [9, 9, 9], [9, 9, 9, 9, 9]]

[DOO²Q#ćD_Piˆëć<¸«¸ì]
We then iterate over these lists decrementing elements in a list until the list consists only of zeroes, then moving on to the next. We stop when the total sum equals the second input.

[     #              ]   # loop until
 DOO                     # the sum of the list of lists
    ²Q                   # equals the second input
        ć                # extract the first list
         D_Pi            # if the product of the logical negation of all elements is true
             ˆ           # add the list to the global list
              ë          # else
               ć<        # extract the head and decrement it
                 ¸«      # append it to the list
                   ¸ì    # and prepend the list to the list of lists

Now we need to unsort the final list simulating removing elements from alternating sides instead of left-to-right as we have been doing.

ć                   # extract the row we need to sort
 ā<                 # push a list of indices of the elements [0 ... len(list)-1]
   ΣÉ}              # sort it by even-ness
      2äćR¸ì˜       # reverse the run of even numbers
                    # the resulting list will have 0 in the middle,
                      odd number increasing to the right and
                      even numbers increasing to the left
             è      # index into our final row with this
              ¸ì¯ì  # reattach all the rows to eachother

Now we format the output correctly

J              # join list of lists to list of strings
 DSOIα         # calculate the absolute difference of sum of our triangle and the 3rd input
      '-‚ì     # pair it with the string "-" and append to the list of rows
          .C   # join by newlines and center each row